In the ongoing development of communications services, new systems and techniques such as distributed antenna networks are being investigated. In a distributed antenna network, wireless communication is provided from a central location using multiple remote antenna units. One known type of a distributed antenna network is a multicast network, an example of which is illustrated in FIG. 1 where a network coverage area includes multicast cells 20.sub.1, 20.sub.2, . . . 20.sub.n and smaller multicast subcells 25.sub.1, 25.sub.2, 25.sub.3, . . . 25.sub.z. Each subcell is served by a remote antenna unit 21.sub.1, 21.sub.2, 21.sub.3, . . . 21.sub.z. The remote antenna units are each connected to one of a plurality of transceivers 10.sub.1, 10.sub.2, 10.sub.3, . . . 10.sub.n contained in a transceiver unit 10. Each transceiver operates on a different frequency or set of frequencies. The transceivers 10.sub.1, 10.sub.2, 10.sub.3, . . . 10.sub.n are respectively connected to one or more of the remote antenna units 21.sub.1, 21.sub.2, 21.sub.3, . . . 21.sub.z. Each multicast cell is served by a single transceiver, and each subcell is served by a single antenna unit. All of the subcells within a cell are served by the same transceiver, and thus the same frequency or frequency set. Since each transceiver can serve multiple multicast subcells, multicasting allows each transceiver 10.sub.1, 10.sub.2, 10.sub.3, . . . 10.sub.n to cover a larger geographic area 20 than normally provided by a single antenna unit.
Mobile communication systems are known or under development which are capable of providing high quality signals at lower costs than traditional cellular infrastructures. For example, communication networks presently exist which have been designed from cable television (CATV) infrastructures. One particular system uses a hybrid fiber/coax (HFC) CATV infrastructure to increase communication capacity and improve service quality. Although it is theoretically possible for any CATV infrastructure to support a mobile communication network, the HFC cable infrastructure is considered to be an economical alternative to wireless providers seeking to avoid the high cost of network construction.
FIG. 2 illustrates the basic components of a CATV infrastructure used to support wireless communications. In FIG. 2, base stations 50.sub.1 and 50.sub.2 are connected to a public network such as a public switched telephone network. Remote antenna signal processors RASP1 52.sub.1 and RASP2 52.sub.2 connect the base stations 50.sub.1 and 50.sub.2 to a fiber equipment 54. The fiber equipment 54 is connected to a fiber node 58 by fiber optic cable 56 and the fiber node 58 is connected to remote antenna driver (RAD) nodes 62.sub.1 and 62.sub.2 by two-way coaxial cable 60. The RAD nodes 62.sub.1 and 62.sub.2 each include a group of RADs 64.sub.1 and 64.sub.2 and 66.sub.1 and 66.sub.2, respectively, connected to antennas 68.sub.1, 68.sub.2, 70.sub.1 and 70.sub.2, respectively. Each RASP converts radio frequency signals into CATV frequency signals for transmission over the CATV system and each RAD converts CATV frequency signals back into radio frequency signals. More specifically, the RASPs 52.sub.1 and 52.sub.2 convert the radio frequency signals from the base stations 50.sub.1 and 50.sub.2 and then transmit the converted signals in the downlink path toward fiber node 58 and coaxial cable 60. It should be appreciated that RADs and RASPs are sometimes referred to as cable microcell integrators (CMIs).
The RADs 64.sub.1, 64.sub.2, 66.sub.1, and 66.sub.2 receive radio frequency signals and convert these signals into CATV signals suitable for uplink transmission in the CATV system. The RASPs 52.sub.1 and 52.sub.2 convert the upstream CATV frequency signals back into radio frequency signals for processing by base stations 50, and 502. This CATV infrastructure also may accommodate equipment for multiple modulation schemes, such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA).
In distributed antenna networks where wireless communication service is provided from a centralized location using remote antenna units, the multiple remote antenna units are tuned to the same transmit and same receive carrier frequencies to create multicast cells. Because all remote antenna units in a multicast cell are connected to the same radio transceiver unit, time delays can occur due to the variations in the lengths of the transmission paths and/or types of transmission media connecting the remote antenna units.
FIG. 3 illustrates the problems associated with transmission time delay for a variety of transmission paths in a distributed antenna network. In this example, remote antenna units 21.sub.1, 21.sub.2, 21.sub.3, and 21.sub.4 operate on the same radio frequency but in different multicast subcells. These antenna units are connected with a transmission media interface 32 by transmission paths 31.sub.1, 31.sub.2, 31.sub.3, and 31.sub.4, respectively. A transceiver unit 10 connected to media interface 32 communicates with a mobile unit 35 over this network. Each of these transmission paths 31.sub.1, 31.sub.2, 31.sub.3, and 31.sub.4 has a different length, resulting in different delay times for transmitted signals. If the transmission paths are similar in length so that the signals are received at the transceiver unit 10 within a certain time window, the transceiver 10 can compensate for the time delay by using an equalizer. However, if the lengths of the transmission paths are sufficiently different, identical signals transmitted over different transmission paths will arrive at the transceiver 10 with delay differences greater than the acceptable time window, and multicast interference will occur. The degree of multicast interference depends on numerous factors, but is influenced greatly by delay differences due to the various lengths of the transmission paths and signal strength differences between the different paths. The communication problems caused by multicast interference are identical to intersymbol interference experienced in conventional radio communication systems; however, the source of multicast interference is different from that of normal cellular intersymbol interference. Existing solutions do not adequately reduce multicast interference.
FIGS. 4(a)-4(c) illustrate the equalizing of signals received by a transceiver equalizer at various times due to different length transmission paths. In FIGS. 4(a)-4(c), a differential delay time .DELTA.t represents the time difference between signals received at a transceiver over two different transmission paths. In FIG. 4(a), remote antenna units 21.sub.1 and 21.sub.2 corresponding to multicast subcells 25.sub.1 and 25.sub.2 are connected to the transceiver 10 by transmission paths 31.sub.1 and 31.sub.2, respectively. Multicast subcells 25.sub.1 and 25.sub.2 are served by the same radio frequency. Mobile 35 is at the boundary between the multicast subcells 25.sub.1 and 25.sub.2, so that the mobile 35 is approximately equidistant from the remote antenna units 21.sub.1 and 21.sub.2. The strengths of the signals received at the remote antenna units 21.sub.1 and 21.sub.2 are therefore substantially equal if the path lines to each remote antenna unit are similarly situated. However, while mobile 35 is approximately equidistant from antenna units 21.sub.1 and 21.sub.2, transmission paths 31.sub.1 and 31.sub.2 are of different lengths, causing a differential delay time .DELTA.t between identical signals received at a transceiver. In FIG. 4(b), the differential delay time .DELTA.t is within the time window of the transceiver equalizer. In this case, the equalizer is able to compensate for this transmission delay and prevent the detrimental effects of multicast interference. However, when the differential delay time .DELTA.t is greater than the time window of the equalizer, as shown in FIG. 4(c), intersymbol interference occurs and quality is degraded. While it is possible to design an equalizer that compensates for a longer differential time delay, this approach is typically not practical.
Techniques are known for adjusting the transmission delays for transmission paths of various lengths so that the delay times for multicast cells are equalized to prevent multicast interference. Such techniques are described, for example, in U.S. application Ser. No. 08/683,382, entitled "Distributed Antenna Delay Compensation", filed on Jul. 18, 1996. According to the disclosed technique, specialized equipment can be used for detecting the delay of each separate transmission path and adjusting the delay factors so that the associated delay time is equalized. It would be desirable to reduce multicast interference in an economical and efficient manner by eliminating the need for specialized equipment and labor-intensive procedures.